AC ducts in attic accumulate water in winter

I have posted this question in the past, but never received a
satisfactory answer. Perhaps this time someone smarter than I can
come up with a good solution...
Question: How to stop AC ducts in the attic from accumulating water
during the winter
Environment: Ranch house, baseboard hot water heating, whole house
air conditioner with AC registers in the ceiling. Floor of attic
insulated to at least R40. Ceiling of attic not insulated. AC air
handler in attic attached to rigid duct surrounded by 4 inches of
insulation. Rigid duct traverses the center of the attic for about
2/3 the length of the house. Flexible, insulated 6 inch duct rated
R4.2 attaches the ceiling registers to the rigid duct. AC ceiling
registers have internal dampers that can be closed in the winter.
Problem: When the AC ceiling registers are closed for the winter, the
internal dampers do not affect a perfect seal. Some warm, moist air
from the house will leak through the ceiling registers into the
flexible duct and into the rigid duct. Even though the rigid duct is
insulated, on very cold winter days, the internal temperature of the
flexible and rigid ducts will be low enough to condense the water
vapor in the warm air that leaked from the house into the ducts. This
condensed water accumulates in the rigid duct, causing rusting and if
there is enough water, leaking back into the house through the ceiling
registers.
My solution: Not perfect. Every fall, I enter the attic and
disconnect the flexible ducts from the ceiling registers. I cover the
ceiling registers with lots of insulation. In the late spring, I go
into the attic and reverse this process by reconnecting the flexible
ducts to the ceiling registers. This process is a royal pain. I
would like a more permanent solution.

Other than taping saran wrap over the grills, how easy would it be to
slip something behind the grills?
If they are plastic and held in place with 4 screws like mine, remove
one or more screws and loosen the others just enough so you can slip
something inside to block the vent, like a piece of cardboard, or other
thin material, cut to the shape of the vent grill.

Good question, I struggle with this also. One option I have actually
done. Leave the registers open and heat the whole AC ductwork. Kinda
Pricey because the insulation is not that thick, not to mention the
vapor barrier is on the wrong side.

A decent amount of work, also the way my registeres are anchored, this
is a two person job. 'There must be an easier way'(?).

I admire your dedication, but this is a lot of work!

Ahhh, I had a sheet magnet and tried this on one register, only to
find it was not magnetic!

Then how about constructing some sort of cover made from a thin sturdy
material and use clips (I'm thinking plastic automobile type panel push
clips) that would push into the slots in the vent. With the right size
clips, it would be an easy snap-on snap-off arrangement. ( I wonder if
someone doesn't make these already?).
Google on - ac vent covers -, you'll find some other ideas like,
http://www.energyfederation.org/gwne/default.php/cPath/86_742_111

Thank you all for your suggestions. Let me add additional details
about the registers in the ceiling. The resiters are not the
rectangular type on which you can attach the magnetic type covers.
Rather the registers are circular, that is, there are 4 concentric
round metal rings. In the center there is a know that opens/closes
the internal dampers. While the rectangular registers are somewhat
flush with the ceiling, the circular registers protrude about 2 inches
and their construction prevents a cover from being attached to the
register to close the opening.

Then my only solution would be my first, and that was to tape clear
plastic wrap, or whatever, over the vent. You can use the blue 3M
painters tape which claims not to leave a residue when taken off. The
blue stripe around the vent should add a nice contrasting accent color
to the rooms. :-)

Close the dampers with the knob. Cut some overlapping (1/3 round in
my case) partial circles out of duct wrap and slid them in over the
braces holding the knob (or, in other words, next to the dampers) and
see if that works. Once cut to the proper size, you could always pull
them and spray paint the aluminum foil some appropriate color---then
reinstall.
Replacing several feet of the duct closest to the register (or the
entire duct) with flexible duct should also work---may be more than
you want to do.

Actually the duct that connects to the register is flexible and
insulated (R4.2). THis makes it fairly easy to disconnect the duct
work in the fall and reconnect in the spring. I am thinking ahead to
when I am old and not as nimble as I am today and do not want to crawl
around in the attic. Which is why I am looking for a solution that
involves doing something in the house and not the attic.

If the louvers don't stick out (too much?), and are steel, maybe you
can get some of that magnetic sheeting, such as are used on truck
doors to temporarily label a truck with the business name, and put
them over the registers every fall. Are brass registers steel
underneath, or are they solid brass? You can tell with a magnet.

Replace this with aluminum? That will cause more cooling loss when
cooling the house with a hot attic, but you have insulation around it
anyhow.

Meirman
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or not you are posting the same letter.
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